1. Freezing cold winds run rampant across the Northeast, the brittle branches shivering under the hapless sun. It's real cold, too. This map shows the surface flow this morning. The strongest winds are usually where the isobars are closest (the rule is best over flat land and over water).

2. Strong winds will make the cold feel fierce tonight in the Northeast. Heavy lake-effect snow this afternoon and tonight will create narrow bands of whiteout blizzard conditions. The wind will diminish tomorrow (still strong in the morning in New England. The lake-effect snow gun will be silenced.

3. Very fast slow aloft mean fast-changing weather from tomorrow right through the weekend. A low pressure area will spread light snow into Chicago tomorrow. Most of the accumulation should be between tomorrow afternoon and Saturday morning, with another round between late Saturday and early Sunday. The snow may mix with sleet or freezing rain for a while as the temperature climbs toward 30 Saturday. However, by time the Bears play on Sunday, the temperature will be zero.

4. Along the I99, I81 and I95 corridors, snow will spread across Maryland, Pennsylvania, Delaware and New Jersey late tomorrow night. It may fall heavily enough for a short while to make people think a snowstorm is starting. However, warm air aloft will stream in shortly thereafter, changing the snow to sleet and then rain at the coast and to sleet and freezing rain farther inland. The transition to plain rain will happen within an hour or two near I-95 but will take several hours longer along much of I-81.

Even up past Watertown, New York, and Portland, Maine, there should be a change to rain after snow accumulates for a while. It may barely get above freezing in the I-99 corridor until the end of the day tomorrow, so the threat of freezing rain and sleet lasting at least six hours is greatest there. The amount of precipitation to expect is uncertain in various areas because there may be dry patches embedded in the storm.

West of the Appalachians In the I-79 and I 71 corridors, little, if any, snow or ice is likely south of their intersections with I-70, but from I-80 on north, there could be several hours of snow and sleet before the mild air surges in. Temperatures could pass 50 degrees in Cleveland late Saturday then sink to the 20s by Sunday morning. In the areas getting heavy lake-effect snow today and tonight, the snow on the ground may absorb much of the rain, but there could be enough warmth and water around for ponding on roads and temporary local flooding.

From the Virginias up through Maryland and the central and eastern parts of Pennsylvania and New York state there could be a period of heavy rain late Saturday night into part of Sunday ahead of the next cold front. There may be a change snow behind the cold front, with the best chance for some accumulation in western Pennsylvania and central and western New York.

These maps show the storm system at several times during the weekend. On each map, the precipitation shown is for the six-hour period leading up to the time on the map. As a rule of thumb, the snow-rain line should be about midway between the southernmost blue dashed line and the northernmost red dashed line. (the invisible 5,430-meter 1000-500 mb thickness line). The maps represent ensemble means from the 1 a.m. ET GFS run. Quick inspection of the 12Z GFS deterministic run showed good agreement in most areas.

On this map, the snow-rain line is at C, freezing rain and or sleet should be occurring at B, and rain and drizzle with fog draping the mountains and D near Chicago is near the snow-rain line.